The present invention relates to a grating coupled surface emitting device for extracting output light by a diffraction grating of the second or higher order in a direction perpendicular to the resonator direction and, more particularly, to a semiconductor laser and a semiconductor optical amplifier.
In recent years, to realize a fiber-optic subscriber system, many studies have been made for the development of low-cost light transmission/reception terminal devices. To realize a low-cost light transmission/reception terminal device, a semiconductor laser serving as a transmission light source must be directly coupled to an optical fiber without using any lens to decrease the number of components. However, since a conventional waveguide semiconductor laser and optical fiber are greatly different in spot size, the coupling efficiency is very low. In a semiconductor laser in which a spot size conversion waveguide is integrated, although a high optical coupling efficiency is obtained, the margin for the alignment precision between the semiconductor laser and the optical fiber is insufficient.
On the other hand, various optical devices for information processing have extensively been developed along with an increase in transmission information amount. Of these optical devices, a surface emitting device receives a great deal of attention because of simple parallel processing. This surface emitting device is also characterized in that optical coupling is easy due to a relatively large beam diameter. Particularly in short wavelength bands, a high-performance surface emitting laser having a sub-mA oscillation threshold.
In long wavelength bands useful in optical communication, no satisfactory surface emitting laser is reported owing to material limitations, such that a high-reflectivity mirror cannot be formed because no material system that has a large refractive index difference is available, and the materials have many non-emission components unique to themselves. For this reason, it is desired to realize a surface emitting laser having excellent features, like an edge emitting laser used in optical communication and optical interconnection.
Although a grating coupled surface emitting laser is also examined, this laser has a problem in which the output radiation-mode light has two peaks in the waveguide direction. To solve this problem, integrating a plurality of phase shift structures in the waveguide is proposed. However, the emission pattern of the radiation-mode light in this case is a rectangular pattern, which is greatly different from a Gaussian distribution as the native mode of a fiber, resulting in a very low coupling efficiency and a small tolerance against axis shift or axial misalignment.
Recently, application of a surface emitting device to parallel optical information processing and a signal connection bus line between CPUs is examined. Connecting multiple surface emitting devices requires a surface emitting semiconductor optical amplifier for preventing attenuation of light serving as a signal. Although many examples of the surface emitting semiconductor laser are reported, few examples of the surface emitting semiconductor optical amplifier are reported at present.
Report examples about the surface emitting semiconductor optical amplifier are few as the surface emitting semiconductor optical amplifier is essentially difficult to realize due to its structure, compared to a waveguide semiconductor optical amplifier. This is because, in the surface emitting semiconductor optical amplifier, an active layer serving as a gain medium which amplifies light must be made sufficiently thick. Since an active layer having a thickness of only about several microns can be stacked on a semiconductor substrate with the current crystal growth technique, a surface emitting optical amplifier capable of obtaining a sufficient amplification factor is very difficult to realize.
In this way, the use of a surface emitting laser as the transmission/reception light source of a light transmission/reception terminal device of a fiber-optic subscriber system has conventionally been examined. In a laser of this type, although the beam diameter can be increased, the oscillation threshold becomes high in long wavelength bands to weaken the optical output. In a grating coupled surface emitting laser, the emission pattern of the radiation-mode light is rectangular, which is greatly different from a Gaussian distribution as the native mode of an optical fiber. For this reason, the coupling efficiency with the optical fiber is very low, and the tolerance against axis shift is small.
In a known traveling wave type surface emitting semiconductor optical amplifier, a sufficient amplification factor cannot be obtained because the active layer is difficult to make thick.